Method of Pumping Gas

ABSTRACT

A method of pumping a gas stream containing a condensable species and a light gas and an apparatus therefore, the method comprising the steps of conveying the gas stream to a multistage vacuum pump comprising an inlet stage and an exhaust stage downstream from the inlet stage, and adding to the gas stream upstream of or at the inlet stage a purge gas heavier than said light gas to both inhibit migration of the light gas from the exhaust stage towards the inlet stage and to inhibit condensation of the condensable species within the pump.

The present invention relates to a method of pumping gas, and in particular to a method of pumping a gas stream comprising a light gas, such as hydrogen, and a condensable species.

A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, process gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate.

Group III-V compound semiconductors are generally formed using a form of CVD usually known as MOCVD (metal organic chemical vapour deposition). In overview, this process involves reacting together volatile organometallic sources of the required group III with group V elements supplied from hydride gases such as AsH₃ and PH₃. For example, a thin film of GaInP may be formed on a suitable substrate material by supplying to a process chamber trimethyl gallium ((CH₃)₃Ga) as the source of gallium, trimethyl indium ((CH₃)₃In) as the source of indium, and phosphine (PH₃) as the source of phosphorus. Hydrogen gas is generally also present, providing a carrier gas for the organometallic sources and any other process gases and to complete the reaction chemistry by mopping up dangling bonds.

In such deposition processes, the residence time of the deposition gases in the processing chamber is relatively short, and only a small proportion of the gas supplied to the chamber is consumed during the deposition process. Consequently, much of the deposition gases supplied to the chamber is exhausted from the chamber together with by-products from the deposition process.

If the unconsumed process gas or by-product is condensable, condensation on lower temperature surfaces can result in the accumulation of powder or dust within a vacuum pump used to draw the exhaust gases from the process chamber. For instance, in the above example, the gas exhaust from the chamber will typically include relatively large amounts of hydrogen, phosphine. and phosphorus vapour, and so there is a risk that the phosphorus vapour may condense as solid phosphorus within the pump and, over time, effectively fill the vacant running clearance between the rotor and stator elements of the pump, leading to a loss of pumping performance and ultimately pump failure.

Condensation of solid matter within the pump is also a potential problem associated with the evacuation of a process chamber in which an epitaxial deposition process is being conducted. Epitaxial deposition processes are increasingly used for high-speed semiconductor devices, both for silicon and compound semiconductor applications. An epitaxial layer is a carefully grown, single crystal silicon film. Epitaxial deposition utilizes a silicon source gas, typically silane or one of the chlorosilane compounds, such as trichlorosilane or dichlorosilane, in a hydrogen atmosphere at high temperature, typically around 800-1100° C., and under a vacuum condition. Epitaxial deposition processes are often doped with small amounts of boron, phosphorus, germanium, arsenic, or carbon, as required, for the device being fabricated. Hydrogen chloride may also be used to clean the chamber between deposition runs. By-products of the epitaxial process tend to be compounds of silicon and chlorine, or compounds of silicon and hydrogen. These by-products may include chlorosilane polymers of the form Si_(x)Cl_(y)H_(z). These polymers can be converted to self-ignitable or explosive materials, for example polysiloxanes, if exposed to moisture contained in the atmosphere. Consequently, if this material is allowed to accumulate within the pump, there is a risk that when the pump is exposed to air, for example if the foreline used to convey the gases from the process chamber to the pump is dissembled during maintenance or due to the accidental ingress of air into the foreline due to a leakage in the foreline, an explosion may occur.

In view of this, it is common practice to use one or more traps upstream from the pump to remove solid particulates from the gas stream before it enters the pump. These traps require frequently servicing for emptying and cleaning purposes, typically every few days, and this can incur costly downtime of the process tool. Another alternative is to heat the pump using an external heater to a temperature above that at which the solid matter condenses within the pump. However, such heaters tend to be expensive.

It is an aim of at least the preferred embodiment of the present invention to seek to provide an improved method of pumping a gas stream containing both a light gas such as hydrogen and condensable species, such as phosphorus vapour or polymeric vapours.

In a first aspect, the present invention provides a method of pumping a gas stream containing a condensable species and a light gas, the method comprising the steps of conveying the gas stream to a multistage vacuum pump comprising an inlet stage and an exhaust stage downstream from the inlet stage, and adding to the gas stream upstream of or at the inlet stage a purge gas heavier than the light gas to both inhibit migration of the light gas from the exhaust stage towards the inlet stage and to inhibit condensation of the condensable species within the pump.

Light gases such as hydrogen are both difficult to compress and prone to migrate back from the high pressure exhaust stage of the pump towards the low pressure inlet stage, effectively reducing the pumping speed of the gas stream. By adding purge gas such as nitrogen to the gas stream, the larger, heavier nitrogen molecules can mix with the smaller, lighter hydrogen molecules in the gas stream to raise the average molecular size of the gas stream within the pump, which tends to inhibit the hydrogen migration towards the inlet stage and enhance the hydrogen pumping.

The introduction of the purge gas into the gas stream also serves to reduce the partial pressure of condensable species, such as phosphorus vapour, ammonium chloride or polymeric vapours. Furthermore, by adding the purge gas to the gas stream upstream of or at the inlet stage of the pump, the purge gas is compressed with the gas stream as it passes through the pump from the inlet stage to the exhaust stage, generating heat. The additional heat generated by the compression of the purge gas serves to increase the temperature of the gas stream as it passes through the pump which, coupled with the reduction in partial pressure of the condensable species due to the presence of the purge gas, serves to inhibit condensation of the condensable species within the pump without the need to provide any external heating systems.

The pump preferably has a relatively high volume ratio, that is, the ratio between the volume of the inlet stage of the pump and the volume of the exhaust stage of the pump is relatively high. For example, the exhaust stage may have a volume at least three times smaller than that of the inlet stage, more preferably a volume at least five times smaller than that of the inlet stage. By introducing purge gas into the gas stream at or upstream from the inlet stage of a pump having a relatively high volume ratio, over-compression of the gas stream can occur at an early stage in the passage of the gas stream through the pump so that the gas stream becomes rapidly heated.

We have found that when the light gas has a flow rate of at least 20 slm, the purge gas is preferably added to the gas stream at a rate of at least 10 slm to provide the aforementioned benefits. Therefore, in a second aspect the present invention provides a method of pumping a gas stream containing a condensable species and a light gas having a flow rate of at least 20 slm, the method comprising the steps of conveying the gas stream to a multistage vacuum pump comprising an inlet stage and an exhaust stage having a volume at least three times smaller than the inlet stage, and adding to the gas stream upstream of or at the inlet stage a purge gas heavier than said light gas at a rate of at least 10 slm. For example, when the light gas has a flow rate of at least 50 slm, the purge gas is preferably added to the gas stream at a rate of at least 20 slm.

In addition to the purge gas added to the gas stream at or upstream from the inlet stage of the pump, purge gas may also supplied to the pump between stages of the pump. For example, where the pump has five stages, purge gas may be supplied between the third stage and the fourth stage of the pump, and/or between the fourth stage and the fifth (exhaust) stage of the pump without condensation of condensable species within the pump. For example, when the light gas has a flow rate of at least 50 slm, the further purge gas is preferably supplied at a flow rate of at least 20 slm. “Splitting” the introduction of the purge gas along the pump in this manner can enable a lower pressure gas stream to be pumped.

This invention is particularly suitable for use with a multistage vacuum pump comprising at least one, preferably a plurality of pumping stages which compress the gas stream, such as Northey (or “claw”) pumping stages. Northey pumping stages are essentially self-valving, in that the back-migration of gaseous species such as nitrogen towards the pump inlet is inhibited by the Northey pumping mechanism, and so the introduction of purge gas at locations between stages of the pump does not affect the pressure at the inlet of the pump.

In a third aspect the present invention provides a vacuum pumping arrangement for pumping a gas stream containing a condensable species and a light gas, the pumping arrangement comprising a multistage vacuum pump comprising an inlet stage and an exhaust stage downstream from the inlet stage, and a purge gas supply for adding to the gas stream upstream of or at the inlet stage a purge gas heavier than the light gas to both inhibit migration of the light gas from the exhaust stage towards the inlet stage and to inhibit condensation of the condensable species within the pump.

In a fourth aspect, the present invention provides a. vacuum pumping arrangement for pumping a gas stream containing a condensable species and a light gas having a flow rate of at least 20 slm, the pumping arrangement comprising a multistage vacuum pump comprising an inlet stage and an exhaust stage downstream from the inlet stage and having a volume at least three times smaller than the inlet stage, and a purge gas supply for adding to the gas stream upstream of or at the inlet stage a purge gas heavier than the light gas at a rate of at least 10 slm.

Features described above in relation to any of the first and second aspects of the invention are equally applicable to any of the third and fourth aspects, and vice versa.

Preferred features of the present invention will now be described with reference to the accompanying drawings, in which

FIG. 1 illustrates a multistage vacuum pump; and

FIG. 2 illustrates the variation of the partial pressure of phosphor with temperature (i) when purge gas is supplied only to the exhaust of a multistage vacuum pump and (ii) when some of the purge gas is supplied to the inlet stage of the pump of FIG. 1.

With reference first to FIG. 1, a multistage vacuum pump 10 comprises a stator 12 housing a multistage rotor assembly 14. The stator 12 comprises a plurality of transverse walls 16 which divide the stator 12 into a plurality of pumping chambers. In this example, the stator 12 is divided into five pumping stages, although the stator 12 may be divided into any number of pumping stages required to provide the pump 10 with the desired pumping capacity.

The rotor assembly 14 comprises two intermeshing sets of rotor components 18, 20, 22, 24, 26, each set being mounted on a respective shaft 28, 30. The rotor components may have a Roots or a Northey (or “claw” profile). For example, the rotor components 18 may have a Roots profile, with the other rotor components 20, 22, 24, 26 having a Northey profile. The sets of rotor components are preferably profiled in order to maintain a small running clearance between the faces of the rotor components and the facing surfaces of the stator 12.

Each shaft 28, 30 is supported by bearings 32, 34 for rotation relative to the stator 12. The shafts 28, 30 are mounted within the stator 12 so that each pumping chamber houses a pair of intermeshing rotor components, which together provide a stage of the pump 10. A motor 36 is connected to one end of shaft 28. The other shaft 30 is connected to shaft 28 by means of meshed timing gears 38 so that the shafts 28, 30 are rotated synchronously but in opposite directions within the stator 12. In this example, the gears 38 are formed from magnetic material.

A pump inlet 40 communicates directly with the inlet pumping stage, which comprises rotor components 18 and pump exhaust 42 communicates directly with. the exhaust pumping stage, which comprises rotor components 26. Gas passageways 44, 46, 48, 50, 52 are provided within the pump 10 to permit the passage therethrough of pumped gas from the inlet 40 to the exhaust 42.

In order to achieve a reduced pressure at the inlet 40 of the pump 10, the volume of the pumping chambers defined within the stator 12 decreases from the inlet pumping stage, which includes rotor components 18, to the exhaust pumping stage, which includes rotor components 26. For example, the exhaust stage may have a volume at least three times smaller than that of the inlet stage, more preferably a volume at least five times smaller than that of the inlet stage.

The pump 10 is particularly suitable for pumping a gas stream containing a light gas, such as hydrogen, and one or more condensable species, such as phosphorus vapour, ammonium chloride or polymeric vapours, typically output from a semiconductor or flat panel process chamber. Due to the nature of the gases contained in the gas stream, a purge gas supply system 60 is provided for supplying purge gas, for example, nitrogen, to the pump 10. In this example, the purge gas supply system 60 comprises a manifold 62 having an inlet 64 and a plurality of outlets 66. The inlet 64 is connected to a source of purge gas, typically via a conduit including a check valve and optionally a pressure regulator for controlling the pressure of a stream of purge gas conveyed to the inlet 64. Within the manifold 62, the received stream of purge gas typically passes through a mass flow transducer before being split into a plurality of streams for conveyance to the outlets 66. As the flow requirement at each outlet 66 may be different, the manifold 62 may contain an arrangement of solenoid valves, fixed flow restrictors and/or variable flow restrictors for adjusting the flow rate of each stream of purge gas supplied to an outlet 66.

The purge gas supply system 60 comprises a number of pipes each for conveying purge gas from a respective outlet 66 of the manifold 62. One of the pipes 68 a conveys purge gas to a conduit 70 for conveying a gas stream to the inlet 40 of the pump 10 so that the purge gas mixes with the gas stream upstream of the inlet pumping stage. As an alternative, the pipe 68 a may be configured to supply purge gas to a purge port of the pump 10 so that the purge gas is supplied directly to the inlet pumping stage, or to another location upstream from the inlet pumping stage. A second pipe 68 b and a third pipe 68 c convey purge gas to respective purge ports 72, 74 provided about the pump 10. In this example, the purge port 72 is located between the third and fourth pumping stages of the pump 10, and the purge port 74 is located between the fourth and fifth (exhaust) pumping stages of the pump 10. Whilst in this example the pump 10 includes two purge ports, the pump 10 may be provided with any number of purge ports located at various locations about the pump 10.

By conveying purge gas into the gas stream at least upstream from the inlet pumping stages, the purge gas can perform the dual roles of (i) inhibiting migration of the light gas from the exhaust pumping stage towards the inlet pumping stage and (ii) inhibiting condensation of the condensable species within the pump 10. The results of experiments to illustrate this will now be explained with reference to FIG. 2.

In one experiment, a gas stream containing 70 slm of hydrogen and 1 slm of phosphor was conveyed to a five stage Roots pump. A purge gas stream containing 55 slm of nitrogen was conveyed to the exhaust of this pump. In pumps using an all-Roots pumping mechanism, purge gas is typically conveyed to the exhaust so that the purge gas enters the pumping mechanism through back-migration of the purge gas without causing pressure fluctuations at the pump inlet. The phosphor partial pressure and the temperature of the gas stream were recorded at various different locations along the pump, and at the pump inlet and pump outlet. The recorded values were plotted on a graph having the phosphor partial pressure as the vertical axis and the temperature as the horizontal axis, as illustrated at line 78 in FIG. 2. Line 80 in FIG. 2 is the vapour phase equilibrium curve for phosphor, which defines the temperature and partial pressure at which phosphor will condense or sublime. Above the curve, phosphor vapour starts to condense to form as a solid. Below the line, phosphor will remain in the vapour phase. As illustrated by FIG. 2, when the purge gas is supplied to the pump exhaust, the reduction in the temperature of the gas stream typically by 10-20° C. causes the phosphor vapour to condense as phosphor, which could lead to blockage of the pump exhaust and at least the exhaust pumping stage of the pump. In this situation, external heating of the pump and/or of the purge gas would be required to inhibit phosphor condensation, undesirably increasing costs.

In another experiment, a similar gas stream was conveyed to a five-stage pump having a Roots inlet stage and four Northey stages, as illustrated in FIG. 1. A purge gas stream containing 55 slm of nitrogen was also conveyed to this pump. In this experiment, 20 slm of nitrogen was conveyed to the conduit 70 upstream of the inlet stage, 10 slm of nitrogen was conveyed to purge port 72 and 25 slm of nitrogen was conveyed to purge port 74. The phosphor partial pressure and the temperature of the gas stream were again recorded at various different locations along the pump, and at the pump inlet and pump outlet, and the recorded values are illustrated at line 82 in FIG. 2. Due to the compression of the purge gas as it passes through the pump 10, the additional heat generated by this compression serves to increase the temperature of the gas stream by around 10° C. as it passes through the pump 10, maintaining the phosphor in the vapour phase as it passes through and is exhaust from the pump 10.

In addition to inhibiting the condensation of the phosphor within the pump, the additional of nitrogen raises the average molecular size of the gas stream within the pump 10, which tends to inhibit the hydrogen migration towards the inlet stage of the pump 10 and enhance the hydrogen pumping efficiency. 

1. A method of pumping a gas stream containing a condensable species and a light gas, the method comprising the steps of: conveying the gas stream to a multistage vacuum pump having an inlet stage and an exhaust stage downstream from the inlet stage; and adding to the gas stream upstream of or at the inlet stage a purge gas heavier than the light gas, the purge gas being selected so as to inhibit migration of the light gas from the exhaust stage towards the inlet stage and to inhibit condensation of the condensable species within the pump.
 2. The method according to claim 1 wherein the exhaust stage has a volume at least three times smaller than that of the inlet stage.
 3. (canceled)
 4. The method according to claim 2 wherein when the light gas has a flow rate of at least 20 slm, the purge gas is added to the gas stream at a rate of at least 10 slm.
 5. A method of pumping a gas stream containing a condensable species and a light gas having a flow rate of at least 20 slm, the method comprising the steps of: conveying the gas stream to a multistage vacuum pump having an inlet stage and an exhaust stage wherein the exhaust stage has a volume at least three times smaller than the inlet stage; and adding to the gas stream upstream of or at the inlet stage a purge gas heavier than said light gas at a rate of at least 10 slm.
 6. The method according to claim 4 wherein when the light gas has a flow rate of at least 50 slm, the purge gas is added to the gas stream at a rate of at least 20 slm.
 7. The method according to claim 6 wherein the pump comprises at least three stages.
 8. (canceled)
 9. (canceled)
 10. The method according to claim 7 further comprising the step of supplying a further purge gas to the pump between stages of the pump.
 11. The method according to claim 10 wherein when the light gas has a flow rate of at least 50 slm, and the further purge gas is supplied at a flow rate of at least 20 slm.
 12. The method according to claim 11 wherein the vacuum pump comprises at least one pumping stage which compresses the gas stream.
 13. The method according to claim 12 wherein said pumping stage is a Northey pumping stages.
 14. The method according to claim 13 wherein the pump comprises a stator housing a multistage rotor assembly, each stage comprising intermeshing Roots or Northey rotor components.
 15. The method according to claim 14 wherein the pump comprises a plurality of intermeshing Northey rotor components.
 16. A vacuum pumping arrangement for pumping a gas stream containing a condensable species and a light gas, the pumping arrangement comprising: a multistage vacuum pump having an inlet stage and an exhaust stage downstream from the inlet stage; and a purge gas supply for adding to the gas stream upstream of or at the inlet stage a purge gas wherein the surge gas is heavier than the light gas, the purge gas being selected so as to inhibit migration of the light gas from the exhaust stage towards the inlet stage and to inhibit condensation of the condensable species within the pump.
 17. The vacuum pumping arrangement according to claim 16 wherein the exhaust stage has a volume at least three times smaller than that of the inlet stage.
 18. (canceled)
 19. The vacuum pumping arrangement according to claim 17 wherein the purge gas supply is configured to add purge gas to the gas stream at a rate of at least 10 slm when the light gas has a flow rate of at least 20 slm.
 20. A vacuum pumping arrangement for pumping a gas stream containing a condensable species and a light gas having a flow rate of at least 20 slm, the pumping arrangement comprising: a multistage vacuum pump having an inlet stage and an exhaust stage downstream from the inlet stage wherein the exhaust stage comprises a volume at least three times smaller than the volume of the inlet stage; and a purge gas supply for adding to the gas stream upstream of or at the inlet stage a purge gas heavier than the light gas at a rate of at least 10 slm.
 21. The vacuum pumping arrangement according to claim 20 wherein the purge gas supply is configured to add purge gas to the gas stream at a rate of at least 20 slm when the light gas has a flow rate of at least 50 slm.
 22. The vacuum pumping arrangement according to claim 21 wherein the pump comprises at least three stages.
 23. (canceled)
 24. The vacuum pumping arrangement according to claim 22 wherein the purge gas comprises nitrogen.
 25. The vacuum pumping arrangement according to claim 24 wherein the purge gas supply is configured to supply further purge gas to the pump between stages of the pump.
 26. The vacuum pumping arrangement according to claim 25 wherein the purge gas supply is configured to supply the further purge gas to the pump at a rate of at least 20 slm when the light gas has a flow rate of at least 50 slm.
 27. The vacuum pumping arrangement according to claim 26 wherein the vacuum pump comprises at least one pumping stage which compresses the gas stream.
 28. The vacuum pumping arrangement according to claim 27 wherein said pumping stage is a Northey pumping stages.
 29. A vacuum pumping arrangement according to claim 28 wherein the pump comprises a stator housing a multistage rotor assembly, each stage comprising intermeshing Roots or Northey rotor components.
 30. The vacuum pumping arrangement according to claim 29 wherein the pump comprises a plurality of intermeshing Northey rotor components. 